The present invention relates to a system for controlling the air-fuel ratio for an internal combustion engine emission control system with a catalytic converter comprising a three-way catalyst, and more particularly to a system which may temporarily keep the air-fuel ratio at the stoichiometric air-fuel ratio during a high power operating condition of the engine.
Such a control system is a feedback control system, in which the system comprises an oxygen sensor for detecting the concentration of oxygen in the exhaust gases, an air-fuel mixture supply unit, an on-off type electromagnetic valve for correcting the air-fuel ratio of the air-fuel mixture supplied by the air-fuel mixture supply unit, and an electronic control circuit. The electronic control circuit comprises a comparator for comparing the output signal of the oxygen sensor with a predetermined value, an integrating circuit which is connected to the comparator for integrating the output of the comparator, and a driving circuit connected to the integrating circuit for producing driving pulses for driving the on-off type electromagnetic valve. The oxygen sensor generates an electrical signal as an indication of the air-fuel ratio of the air-fuel mixture induced in the engine cylinder.
The output voltage of the oxygen sensor is higher than a predetermined voltage when the oxygen concentration of the exhaust gases is smaller than a predetermined ratio corresponding to the stoichiometric air-fuel ratio in the air-fuel mixture for the combustion of the mixture and is lower than the predetermined voltage when the oxygen concentration is greater than the predetermined ratio. The duty ratio of the driving pulse train varies in dependency on the output of the integrating circuit to correct the air-fuel ratio of the mixture to be supplied to the cylinder to the stoichiometric air-fuel ratio.
In such a system, when the engine is rapidly accelerated, which is called WOT (Wide Open Throttle) condition, the duty ratio is fixed to a predetermined value by the operation of system, in order to provide a rapid acceleration. On the other hand, the air-fuel ratio greatly deviates from the stoichiometric value for 2 or 3 seconds after the wide opening of throttle valve. When the engine operation changes from the transient state to the steady state, the air-fuel ratio reaches the stoichiometry. Therefore, if the duty ratio is fixed to a predetermined value in WOT condition where the controlled air-fuel ratio greatly deviates from the stoichiometry, the purification effect of the emission control system is extremely reduced although the acceleration effect of the engine may be effected.